When the NASA/ESA Hubble Space Telescope observed Mars near opposition on May 12, 2016, Phobos — the larger and inner of the two natural satellites of Mars — photobombed the picture.
On May 12, 2016, Hubble took 13 separate exposures, allowing astronomers to create a time-lapse image showing Phobos during its orbital trek around Mars. This image is a composite of separate exposures acquired by NASA’s Hubble WFC3/UVIS instrument. Image credit: NASA / ESA / Z. Levay, STScI / J. Bell, ASU / M. Wolff, Space Science Institute.
Phobos was discovered by the American astronomer Asaph Hall on August 17, 1877, at the U.S. Naval Observatory in Washington, D.C., six days after he found the smaller, outer moon, named Deimos.
Both moons are named after the sons of Ares, the Greek god of war, who was known as Mars in Roman mythology. Phobos (panic or fear) and Deimos (terror or dread) accompanied their father into battle.
Phobos is approximately 16.2 x 13.7 x 11.2 miles (26 x 22 x 18 km) and has a very lumpy appearance.
It is an unusual satellite, orbiting closer to its planet than any other moon in the Solar System.
The little moon orbits Mars about 3,721 miles (5,989 km) from the surface and completes an orbit in just 7 hours and 39 minutes.
It orbits so close to the Martian surface that the curvature of the planet would obscure its view from an observer standing in Mars’ polar regions.
Its orbital period is about 3 times faster than the rotation period of the Red Planet, with the unusual result among natural satellites that Phobos rises in the west and sets in the east as seen from Mars.
The origin of this strange moon is being studied in detail but remains unclear.
One scenario is that Phobos is a captured asteroid. It is also possible that the tiny moon formed in situ at Mars, from ejecta from impacts on the planet’s surface, or from the remnants of a previous moon which had formed from the Martian accretion disc and subsequently collided with a body from the asteroid belt.
According to planetary researchers, Phobos draws nearer to Mars by about 6.5 feet every hundred years.
They predict that within 20 to 40 million years, it either will crash into the Red Planet or be torn to pieces and scattered as a ring around Mars.
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About two weeks after the Apollo 11 manned lunar landing on July 20, 1969, NASA’s Mariner 7 flew by Mars and took the first crude close-up snapshot of Phobos.
In 1971, NASA’s Mariner 9 became the first spacecraft to orbit another planet, Mars. It carried several scientific instruments for the analysis of the surface and atmosphere of Mars. However bonus science was obtained when these instruments were able to view Phobos and Deimos during close flybys.
On July 20, 1976, NASA’s Viking 1 lander touched down on the Martian surface. A year later, its parent craft, the Viking 1 orbiter, took the first detailed photograph of Phobos.
Recent photos from Mars-orbiting spacecraft reveal that Phobos is apparently being torn apart by the gravitational pull of Mars.
The small moon is marred by long, shallow grooves that are probably caused by tidal interactions with its parent planet.
The NASA/ESA Hubble Space Telescope took the images of Phobos orbiting Mars on May 12, 2016, when the planet was 50 million miles from Earth.
This was just a few days before the planet passed closer to Earth in its orbit than it had in the past 11 years.
Over the course of 22 minutes, Hubble took 13 separate exposures, allowing astronomers to create a time-lapse video showing the diminutive moon’s orbital path.
The Hubble observations were intended to photograph Mars, and the moon’s cameo appearance was a bonus.
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